As a producing oil field matures, the declining reservoir pressures from continued hydrocarbon extraction make oil production from existing and new wells harder. To alleviate this problem in part, natural gas is often injected at high pressure from the casing into the open wellbore of an oil well's string of tubes. This method of artificial lift is known as “gas-lift.” As it is relatively inexpensive, easy to implement, and applicable over a broad range of conditions, it is a favored method of lift in many operating fields. Some or all of the gas produced by a field can be used as the source of lift-gas.
When natural gas is injected at high pressure into the wellbore near the bottom of the well, it mixes with the produced fluids from the reservoir, reducing the density of the fluid column and effectively lowering the bottom-hole pressure. The increased pressure differential induced across the sandface (the connection point between the reservoir and the well) allows more fluid to flow to the surface. However, too much lift-gas increases the frictional pressure drop and reduces the fluid production. Hence, although each well has a desirable lift-gas quantity, when the entire gathering network is considered, an optimal distribution must be made to account for the backpressure effects imposed by interconnected wells. This gives rise to a nonlinear gas-lift optimization problem. Even more broadly, the production also depends on the activation state of wells and the control of subsurface chokes that control flow, among other network elements.
To optimize production, a model of the oilfield must simultaneously optimize values for these different types of control variables. For large-scale network problems, this can be a difficult task when using conventional methods.